1. Field of the Invention
The present invention relates to a heat resistant carburized rolling bearing component and to a manufacturing method thereof.
2. Description of the Background Art
A rolling bearing used in a power transmitting portion or an engine portion of vehicles, airplanes, ships and boats, industrial machines and the like is used under a severe environment. Even under the severe environment, superior rolling fatigue life and reliability are required. In the rolling bearing used for the applications mentioned above, foreign matters such as particles, dusts and iron particles may possibly be involved, and in such an environment, the rolling fatigue life becomes significantly shorter than in a clean environment. As a countermeasure, recently, carbo-nitriding process is performed on high-carbon chromium bearing steel such as SUJ2 or a case-hardened steel such as SCM420 and SNCM815 to generate an appropriate amount of retained austenite immediately below the rolling surface, so as to improve life in an environment where foreign matters are involved.
General carbo-nitriding process, however, requires process time longer than the quenching and tempering process applied to SUJ2, for example, and therefore manufacturing cost of the rolling bearing subjected to the carbo-nitriding process is significantly increased as compared with the rolling bearing manufactured by the common quenching and tempering.
The rolling bearing used for vehicles and airplanes is used in an environment involving not only foreign matters but also high temperature. Therefore, excellent rolling fatigue life is required under very severe conditions of use. Generally, a rolling bearing used in a high temperature environment is prepared by quenching high-carbon chromium bearing steel such as SUJ2 followed by high temperature tempering to attain dimensional stability. High temperature tempering is also performed after carburization and quenching on the case-hardened steel such as SCM420 and SNCM815. When such materials are subjected to high temperature tempering, however, hardness degrades significantly, and therefore, a prescribed hardness required of a rolling bearing cannot be attained, resulting in shorter rolling fatigue life and lower wear resistance. For this reason, a precipitation hardening type steel material such as M50 is used for a rolling bearing used in a high temperature range. However, the cost of the material and manufacturing of such a rolling bearing is high, and in addition, the scope of application is limited, so that it has been impossible to satisfy various needs.
In a rolling bearing subjected to carbo-nitriding process, retained austenite is generated directly below the rolling portion after heat treatment, stress concentration caused by biting of foreign matters is relaxed by the function of the retained austenite, and in addition, resistance to temper softening is improved by nitrogen introduced to the steel, so as to improve rolling fatigue life. As described above, however, a rolling bearing used at a high temperature must be subjected to high temperature tempering, by which the retained austenite is decomposed and the amount thereof is reduced. Therefore the effect mentioned above is not expected. Further, there is a limit to prevent temper softening by the introduced nitrogen, and therefore sufficient performance cannot be attained in an environment involving foreign matters and high temperature.
Recently, development of an engine having high output and small size is in rapid progress in the field of vehicles, for example, and at the same time, the environment of use of the rolling bearing has become increasingly severe. The temperature range at which the rolling bearing is used in the engine portion is generally about 130xc2x0 C. However, instantaneous increase of temperature to 160xc2x0 C. is expected. As the engine comes to have higher output, the temperature range of the rolling bearing will be increased to about 160xc2x0 C. for normal use and up to 200xc2x0 C. or more instantaneously. Therefore, when higher output and reduction in weight of the engine are promoted, it would be impossible to maintain sufficient rolling fatigue life under the expected environment involving foreign matters and the high temperature, by the present high-carbon chromium bearing steel or by the carburizing or carbo-nitriding process.
Further, the precipitation hardening type bearing steel such as M50 is disadvantageous because of the high cost. Therefore, a rolling bearing that is inexpensive and has sufficient rolling fatigue life even under a severe environment of use is desired.
The present invention was made to solve the above-described problems, and an object is to provide a heat resistant carburized rolling bearing component having excellent rolling fatigue life, wear resistance and dimensional stability even under an environment involving foreign matters and an environment of high temperature, and is inexpensive as compared with the prior art examples, as well as to provide a manufacturing method thereof.
Through intensive study, the inventors of the present invention have found combinations and respective contents of composition elements that can provide inexpensive heat-resistant carburized rolling bearing component having excellent rolling fatigue life under the environment involving foreign matters and environment involving high temperature.
The present invention provides a component of a heat resistant carburized rolling bearing having an inner ring, an outer ring and a rolling element, formed of a steel material, at least containing, as alloy elements in a matrix, by mass %, at least 0.1% and at most 0.4% of C (carbon), at least 0.3% and at most 3.0% of Si (silicon), at least 0.2% and at most 2.0% of Mn (manganese), at most 0.03% of P (phosphorus), at most 0.03% of S (sulfur), at least 0.3% and less than 2.5% of Cr (chromium), at least 0.1% and less than 2.0% of Ni (nickel), at most 0.050% of Al (aluminum), at most 0.003% of Ti (titanium), at most 0.0015% of O (oxygen) and at most 0.025% of N (nitrogen) and a remaining part of Fe and an unavoidable impurity, formed by carburizing or carbo-nitriding process followed by quenching, followed in turn by tempering at a tempering temperature of at least 200xc2x0 C. and at most 350xc2x0 C., and having a surface hardness of at least HRC57 after the tempering process.
The heat resistant carburized rolling bearing component of the present invention having the above described composition can attain surface hardness as high as HRC57 or higher even after high temperature tempering, and therefore, satisfactory rolling fatigue life and wear resistance can be attained even under the environment involving high temperature and foreign matters. As high temperature tempering process is performed, retained austenite that is instable to heat can be decomposed in advance, and therefore, dimensional stability at a high temperature environment is ensured.
Further, the steel having the above described composition is inexpensive as compared with the precipitation hardening type bearing steel such as M50.
From the foregoing, it is understood that an inexpensive heat resistant carburized rolling bearing component having excellent rolling fatigue life, wear resistance and dimensional stability under the environment involving foreign matters and the environment involving high temperature can be obtained.
The temperature for tempering process is at least 200xc2x0 C. and at most 350xc2x0 C. A rolling bearing used in a high temperature environment may possibly heated to 200xc2x0 C. or higher. Therefore, tempering process is performed at a temperature not lower than 200xc2x0 C. to ensure dimensional stability. When the temperature for tempering process exceeds 350xc2x0 C., surface hardness will be lower than HRC57, and the life of the rolling bearing abruptly decreases.
By performing carbo-nitriding process in place of carburizing process, it is possible to attain further improved rolling fatigue life, wear resistance and dimensional stability in the environment involving foreign matters and the environment of high temperature.
The reasons why the chemical components of the heat resistant carburized rolling bearing are limited will be described in the following.
(1) C Content (at least 0.1% and at most 0.4%)
C has an influence on core hardness after carburizing or carbonitriding process. In order to ensure the core hardness necessary to attain the required strength of a rolling bearing, it is necessary that C content is at least 0.1%. When the content of C exceeds 0.4%, toughness, susceptibility to hot working and machinability are degraded, and therefore, it is necessary to set the upper limit of C content to 0.4%.
(2) Si Content (at least 0.3% and at most 3.0%)
Si has a function of suppressing softening in a high temperature range and improving heat resistance of the rolling bearing. When Si content is smaller than 0.3%, such effect cannot be attained. Therefore, the lower limit of Si content must be 0.3%. As Si content increases, heat resistance also improves. The effect, however, is saturated when it is added exceeding 3.0%, while susceptibility of hot working and machinability are degraded. Therefore, it is necessary to set the upper limit of Si content to 3.0%.
(3) Mn Content (at least 0.2% and at most 2.0%)
Mn is an element used for deoxidation in manufacturing steel, and at the same time, it is an element that improves quenching property. To obtain such effects, it is necessary to add Mn by at least 0.2%. When the content exceeds 2.0%, however, machinability degrades significantly, and therefore, it is necessary to set the upper limit of Mn content to 2.0%.
(4) P Content (at most 0.03%)
P is segregated at austenite grain boundary of the steel, causing degradation of toughness and rolling fatigue life. Therefore, it is necessary to set the upper limit of P content to 0.03%.
(5) S Content (at most 0.03%)
S hinders susceptibility to hot working of steel and forms a non-metallic inclusion in the steel to degrade toughness and rolling fatigue life. Therefore, it is necessary to set the upper limit of S content to 0.03%. Though S is disadvantageous in the aspect described above, it has an effect of improving machinability. Therefore, though smaller content is desirable, it may be added within the range of at most 0.03%.
(6) Cr Content (at least 0.3% and less than 2.5%)
Cr has the effect of improving quenching property, improving resistance to temper softening and elongating life. In order to attain such effects, the contents must be at least 0.3%. When the content is at least 2.5%, however, large carbide generates, degrading rolling fatigue life.
(7) Al Content (at most 0.050%)
When a large amount of Al exceeding 0.050% is contained, hard oxide inclusion generates, significantly degrading rolling fatigue life. Though Al presents such a problem, it has an effect of making finer the crystal grains by forming AlN. Therefore, Al may be contained by the amount of 0.05% that may not increase manufacturing cost of the steel.
(8) N Content (at most 0.025%)
N has an effect of making finer the crystal grains by forming AlN, bonded to Al. When contained by a large amount, however, strength of the steel is degraded. Therefore, it is necessary to set the upper limit of N content to 0.025%.
(9) Ti Content (at most 0.003%)
Ti forms a nitride to be a non-metallic inclusion, possibly providing a start point of rolling fatigue. Therefore, it is necessary to set the upper limit of Ti content to 0.003%.
(10) O Content (at most 0.0015%)
O forms an oxide in the steel, possibly providing a start point of rolling fatigue as a non-metallic inclusion, resulting in shorter rolling fatigue life. Therefore, it is necessary to set the upper limit of O content to 0.0015%.
(11) Ni Content (at least 0.1% and less than 2.0%)
Ni suppresses change in texture in the process of rolling fatigue when used in a high temperature environment, and it also has an effect of improving rolling fatigue life by suppressing lowering of hardness in a high temperature range. In addition, Ni also has the effect of improving toughness to improve life in the environment involving foreign matters and improving corrosion resistance. To attain such effects, it is necessary to add Ni by at least 0.1%. When the content is at least 2.0%, however, large amount of retained austenite generates at the time of quenching, making it difficult to attain a prescribed hardness and, in addition, the cost of the steel material increases.
Tempering hardness and carbide of the heat resistance carburized rolling bearing will be discussed in the following.
(12) Tempering Hardness
In order to stabilize dimension under the environment of use, it is a general practice that a bearing used in a high temperature range is subjected to tempering at a temperature not lower than the environmental temperature. The inventors conducted detailed study related to the tempering hardness and the rolling fatigue life at the temperature environment of 200xc2x0 C., and as a result, it was found that there was a correlation between the tempering hardness and the rolling fatigue life, and that there was a tendency that the harder the tempering hardness, the longer the rolling fatigue life. Particularly, it was found that when the tempering hardness was the same, a bearing subjected to tempering at a higher temperature had longer life, and even when subjected to high temperature tempering, a bearing having high tempering hardness had longer life. Further, it was found that when surface hardness after tempering was lower than HRC57, the life degraded abruptly, and variation in life increased. In order to improve life at a high temperature and to suppress variations, it is necessary to maintain the surface hardness of at least HRC57, and higher tempering temperature is preferred at this time.
Preferably, in the heat resistant carburized rolling bearing described above, the steel material further includes at least one of Mo (molybdenum) of at least 0.05% and at most 2.5% and V (vanadium) of at least 0.05% and at most 1.0%, both by mass %.
Thus, rolling fatigue life in the environment involving foreign matters and the environment involving high temperature can further be improved, and the hardness after tempering process can be improved.
The reasons why the chemical components described above are limited will be described in the following.
(13) Mo Content (at least 0.05% and at most 2.5%)
Mo has a function of improving quenching property of steel and preventing softening at the time of tempering process as it results in solid-solution in a carbide. Mo is added particularly because it is found to have the function of improving rolling fatigue life in a high temperature range. When a large amount of Mo exceeding 2.5% is contained, however, the cost of the steel material increases and hardness at the time of softening to ease cutting process is not lowered, resulting in significant degradation in machinability. Therefore, it is preferred to set the upper limit of Mo content to 2.5%. When the content of Mo is smaller than 0.05%, the effect in forming carbide is not attained. Therefore, it is preferred to set the lower limit of Mo content to 0.05%.
(14) V Content (at least 0.05% and at most 1.0%)
V is bonded to carbon, precipitating fine carbide, promotes development of fine crystal grains and has an effect of improving strengthxc2x7toughness. Further, V content improves heat resistance of the steel material, suppresses softening after high temperature tempering, improves rolling fatigue life and reduces variations of life. The V content ensuring such effects is at least 0.05%. Therefore, it is preferred to set the lower limit of V content to 0.05%. When a large amount of V exceeding 1.0% is contained, machinability and susceptibility to hot working are degraded. Therefore, it is preferred to set the upper limit of V content to 1.0%.
In the above described heat resistant carburized rolling bearing, preferably, the total content of Mn and Ni in the steel material is at least 1.5%, by mass %. Thus, rolling fatigue life is significantly improved by performing secondary quenching after intermediate annealing, in addition to carburizing or carbo-nitriding process.
The present invention provides a method of manufacturing a heat resistant carburized rolling bearing, having an inner ring, an outer ring and a rolling element, including the following steps. First, a steel material is prepared, that steel material at least including, as alloy elements, at least 0.1% and at most 0.4% of C, at least 0.3% and at most 3.0% of Si, at least 0.2% and at most 2.0% of Mn, at most 0.03% of P, at most 0.03% of S, at least 0.3% and less than 2.5% of Cr, at least 0.1% and less than 2.0% of Ni, at most 0.050% of Al, at most 0.003% of Ti, at most 0.0015% of O and at most 0.25% of N, by mass %, and the remaining part including Fe and an unavoidable impurity. The steel material is subjected to carburizing or carbo-nitriding process followed by quenching. After quenching, the steel material is subjected to tempering at a temperature of at least 200xc2x0 C. and at most 350xc2x0 C.
In the method of manufacturing the heat resistant carburized rolling bearing of the present invention, the steel material having the above described composition is prepared. Therefore, even when it is subjected to high temperature tempering process, a high surface hardness of at least HRC57 can be attained, and therefore satisfactory rolling fatigue life and wear resistance can be attained even under the environment involving high temperaturexc2x7foreign matters. Further, as the amount of retained austenite can be reduced by performing tempering process at a high temperature, dimensional stability in the high temperature environment can be secured.
Further, the steel having the above described composition is inexpensive as compared with the precipitation hardening type steel such as M50.
From the foregoing, it is possible to manufacture an inexpensive heat resistant carburized rolling bearing having superior rolling fatigue life, wear resistance and dimensional stability in the environment involving foreign matters and in the environment involving high temperature.
Preferably, in the method of manufacturing the heat resistant carburized rolling bearing described above, the quenched steel material is subjected to secondary quenching and thereafter subjected to tempering process.
Thus, sufficient surface hardness can be attained.
Preferably, in the method of manufacturing the heat resistant carburized rolling bearing described above, the quenched steel material is subjected to an intermediate annealing, and thereafter subjected to secondary quenching.
Thus, sufficient surface hardness can be attained.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.